Both gas and liquid flow can be measured in volumetric or mass flow rates (such as liters per second or kg/s). These measurements can be converted between one another if the density of the material is known. However, the density of the material is not always known, such as in the case the composition of the fluid is unknown.
Volumetric flow sensors can be embodied as mechanical flow meters including rotometers and pith-ball indicators. Such mechanical flow meters are relatively large mechanical assemblies. Monolithic volumetric flow sensors are available that create a pressure differential from fluid flow normal to a planar stress-gauge diaphragm with a hole in it. Such a sensor, commonly referred to as an orifice plate, is placed in the flow and constricts the flow. Monolithic volumetric flow sensors use the same principle as the venturi meter in that the differential pressure relates to the velocity of the fluid flow (Bernoulli's principle).
Other devices for volumetric flow sensing are based on various configurations for sensing a flow-induced pressure differential. Well known microbridge structures achieve tangential sensing of mass flow only, require a well controlled heater for accuracy, and because of the heater, operate at higher levels of power dissipation. What is needed is new flow sensing devices having reduced complexity for lower cost and/or more robust designs.